Recently, much research has been conducted on the Filter Bank-based Multicarrier (FBMC) communication scheme as one of the next generation communication technologies for transmitting high quality data at a high speed and replacing Orthogonal Frequency Division Multiplexing (OFDM) technology. FBMC is superior to OFDM in terms of low out-of-band radiation and reduction of the number of guard subcarriers while meeting the spectrum mask requirement; and this makes it possible to modulate/demodulate the signals with Cyclic Prefix (CP), resulting in improvement of spectral efficiency and robustness against frequency synchronization error.
The conventional FBMC communication systems may be classified into (1) a transmission/reception method in which a time domain polyphase network (PNN) follows Inverse Fast Fourier Transform (IFFT) and (2) a transmission/reception method in which a frequency domain frequency spreader and overlap/sum structure precedes the IFFT. The technology of (1) implements a time domain convolution operation with a filtering as a sum of the length of M weighted sums and then implements the offset-QAM (OQAM) by applying two PPN modules through a time difference. At this time, the receiver uses a time domain equalizer because the transmitter performs time domain filtering. The technology of (2) performs oversampling and filtering with prototype filters in the frequency domain, IFFT of length KM, and overlapping transmission using an adder and memory. At this time, the receiver uses a one-tap equalizer because the transmitter performs filtering in the frequency domain.
The technology of (2) is described in more detail hereinafter.
FIG. 1 is a block diagram illustrating a transmitter for use in a conventional FBMC communication system, and FIG. 2 is a diagram illustrating a signal flow in the transmitter for use in the conventional FBMC communication system when K=4.
Referring to FIG. 1, the transmission signal d(n) consists of M Offset Quadrature Amplitude Modulation (OQAM) signals d(mM). The OQAM signals are converted by a Serial-to-Parallel (S/P) converter 110, and each OQAM signal di(mM) is spread by a frequency spreader 120 in the frequency domain as shown in FIG. 2. The spreader 120 spreads each OQAM signal into the KM signals in the whole frequency band by multiplying each OQAM signal by 2K−1 frequency domain filter coefficients using the prototype filter. This is called frequency domain filtering.
The filtered signal is IFFT-ed by an IFFT 130. Finally, the output signals of the IFFT 130 are overlapped by a Parallel-to-Serial (P/S) and Overlap/Sum block 140.
In the conventional FBMC system, since the adjacent QAM signals are spread and the results of spreading are overlapped in the spreading process for performing frequency domain filtering, it becomes impossible to recover the signal. In order to overcome this problem, the FBMC system uses OQAM, which arranges the in-phase (real) and quadrature-phase (imaginary) components to cross each other in the tie-frequency resource.
In order to perform frequency domain filtering in the conventional system, the size of the IFFT 130 should be increased as much as K times corresponding to the overlapping factor of the prototype filter in comparison with OFDM, and the total system complexity increases. Since the same problem also occurs at the receiver, the FFT size of the receiver should be increased also as much as K times, and this increases the complexity of the receiver.
FIG. 4 is block diagram illustrating a receiver for use in the conventional communication system.
Referring to FIG. 4, the reception signal x(n) is converted to parallel signals by an S/P converter 210 and then FFT-ed by a Fast Fourier Transform (FFT) 220. Next, the signal is equalized by a frequency equalizer 230 and then frequency-domain-filtered by a frequency de-spreader 240 so as to be recovered. If the QAM signal is used in the frequency domain filtering process as described above, this makes it impossible to cancel the intrinsic interference.